Theory of Steam Generation P M V Subbarao

Theory of Steam Generation P M V Subbarao Professor Mechanical Engineering Department Progressive Development of Power Generation through Steam Generation……

The Theory of Producing Steam • Water boils and evaporates at 100°C under atmospheric pressure. • By higher pressure, water evaporates at higher temperature - e. g. a pressure of 10 bar equals an evaporation temperature of 184°C. • During the evaporation process, pressure and temperature are constant, and a substantial amount of thermal energy is used for bringing the water from liquid to vapour phase. • When all the water is evaporated, the steam is called dry saturated. • In this condition the steam contains a large amount of latent heat. • Further heating of dry saturated steam will lead to increase in temperature of the steam. • Superheated steam.

Steam generator versus steam boiler • Opposite the principle of the steam boilers, the water in the steam generators evaporates inside the tube winded up into serial connected tube coils. • The feed water is heated up to the evaporation temperature and then evaporated. • The intensity of the heat, the feed water flow and the size/length of the tube are adapted, so that the water is exactly fully evaporated at the exit of the tube. • This ensures a very small water and steam volume (content of the pressure vessel). • Thus there are no buffer in a steam generator, and is it temporary overloaded. • The advantages using a steam generator compare to conventional steam boilers.

• Easy to operate - normally no requirement for boiler authorisation • Rapid start-up and establishing full steam pressure Compact and easy to adapt in the existing machinery arrangement • Price attractive - especially at low steam rates. • The advantages using a steam generator compare to conventional steam boilers: • Easy to operate - normally no requirement for boiler authorisation • Rapid start-up and establishing full steam pressure Compact and easy to adapt in the existing machinery arrangement • Price attractive - especially at low steam rates.

Progress in Rankine Cycle Power Generation 1907 1919 1938 1950 1958 1959 1966 1973 1975 MW 5 20 30 60 120 200 500 660 1300 p, MPa 1. 3 1. 4 4. 1 6. 2 10. 3 16. 2 15. 9 24. 1 Th o. C 260 316 454 482 538 566 565 538 T r o. C -- -- 538 566 565 538 FHW -- 2 3 4 6 6 7 8 8 5. 1 4. 5 3. 4 3. 7 4. 4 5. 1 ~17 27. 6 30. 5 35. 6 37. 5 39. 8 39. 5 40 Year Pc, k. Pa 13. 5 h, % --

Steam Generation Theory • Within the boiler, fuel and air are force into the furnace by the burner. • There, it burns to produce heat. • From there, the heat (flue gases) travel throughout the boiler. • The water absorbs the heat, and eventually absorb enough to change into a gaseous state steam. • To the left is the basic theoretical design of a modern boiler. • Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.

• Water enters the boiler, preheated, at the top. • The hot water naturally circulates through the tubes down to the lower area where it is hot. • The water heats up and flows back to the steam drum where the steam collects. • Not all the water gets turn to steam, so the process starts again. • Water keeps on circulating until it becomes steam. • Meanwhile, the control system is taking the temperature of the steam drum, along with numerous other readings, to determine if it should keep the burner burning, or shut it down. • As well, sensors control the amount of water entering the boiler, this water is know as feedwater. • Feedwater is not your regular drinking water. • It is treated with chemicals to neutralize various minerals in the water, which untreated, would cling to the tubes clogging or worst, rusting them. • This would make the boiler expensive to operate because it would not be very efficient.

• • • On the fire side of the boiler, carbon deposit resulting from improper combustion or impurities in the fuel can accumulate on the outer surface of the water tube. This creates an insulation which quickly decrease the energy transfer from the heat to the water. To remedy this problem the engineer will carry out soot blowing. At a specified time the engineer uses a long tool and insert it into the fire side of the boiler. This device, which looks like a lance, has a tip at the end which "blows" steam. This blowing action of the steam "scrubs" the outside of the water tubes, cleaning the carbon build up. Water tube boilers can have pressures from 7 bar to as high as 250 bar. The steam temperature's can vary between saturated steam, 100 degrees Celsius steam with particle of water, or be as high as 600 650 degrees Celsius, know as superheated steam or dry steam The performance of boiler is generally referred to as tons of steam produced in one hour. In water tube boilers that could be as low as 1. 5 t/hr to as high as 2500 t/hr.